The hydrogen of the central mass fuses with helium. While there is an aura of hydrogen, the star will remain in this stage. This is the maturity phase.
The hydrogen of the central mass is exhausted. The force of gravity takes over.
Following the contraction of the star, pressure and temperature are such that the fusion of the helium with carbon begins. The star dilates to evacuate the produced energy. This is the beginning of the final phase.